1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus capable of correcting the record timing of video data based on position deviation of horizontal synchronization signals, and a control method thereof.
2. Description of the Related Art
An electro-photographic type image forming apparatus forms an electrostatic latent image on a photoreceptor by using light beam radiated from a laser scanning unit. The image forming apparatus applies toner to the electrostatic latent image, to form a toner latent image, and then transfers the toner latent image to a printing medium, such that a desired image is printed on the printing medium.
A laser scanning unit outputs horizontal synchronization signals that indicate the start of scanning lines. As the horizontal synchronization signals are supplied to a video controller, video data, corresponding to one scanning line stored in a memory, is supplied to the laser scanning unit. A light source repeatedly turns on and off in correspondence with the supplied video data, so that the image corresponding to the video data is formed.
A positional deviation of horizontal synchronization signals may occur, due to vibrations generated when a laser scanning unit operates. In addition, since a color image forming apparatus can have multiple laser scanning units for different colored toners, the locations of the horizontal synchronization signals, generated according to colored toners, must be precisely aligned to complete the image, by overlapping the different toners. In this regard, a color image forming apparatus prints a test pattern and then performs a color registration, by analyzing the test pattern, to correct the color positions.
A color registration generally must be performed whenever a printing operation is performed for a certain time period, and a process of printing a test pattern on a printing medium, and scanning/analyzing the printed test pattern, should be carried out, to perform the operation. Therefore, there is a problem in that the signal processing load for test pattern analysis is weighted.
In order to solve such a problem, Japanese Unexamined Patent Publication No. 11-55472 discloses a method for counting the scanning time of a laser beam, from a point in time when horizontal synchronization signals are detected by a synchronization detecting sensor, to a point in time when termination signals are detected by a termination detecting sensor. The timing of data recorded on a scanning surface is corrected, according to results obtained by comparing the counted scanning time with a reference value. According to Japanese Unexamined Patent Publication No. 11-55472, the method is simpler than the conventional color registration method. However, since a reference clock with a high frequency should be used to determine a scanning time counted from a synchronization detection time to a scanning termination time, whenever a line is scanned, a process of storing information counted using such a reference clock is required. In addition, components for implementing the process should be added to an apparatus, and a high signal processing load is encountered. Further, the apparatus is uneconomical because a termination detecting sensor is provided to determine a scanning termination, which is in addition to a synchronization detecting sensor.
Japanese Unexamined Patent Publication No. 2004-219849 discloses a method in which a cylindrical lens, having a negative power in the main scanning direction, is additionally installed with an optical sensor to receive horizontal synchronization signals. The lens corrects the timing of data, based on a positional deviation of the horizontal synchronization signals. When the cylindrical lens is used, the important point is to prevent the timing of the data from deviating from a reference, due to the positional deviation of the horizontal synchronization signals, which may occur in each scanning line. Thus, such a method has difficulty in exactly correcting the positional deviation of the horizontal synchronization signals, thereby limiting the output of high-quality images.